1) Field of the Invention
The present invention relates to an operational transconductance amplifier (OTA) that is an amplifier having a controllable Gm (transconductance) and capable of removing a DC offset voltage inside the amplifier, and an automatic gain control (AGC) amplifier capable of obtaining an expanded output dynamic range by using the OTA.
2) Description of the Related Art
By using an OTA that is an amplifier having a controllable Gm (transconductance), an AGC amplifier and a filter each having a fixed gain can be obtained. FIG. 1 shows a circuit diagram of an internal configuration of a conventional OTA. Constant current sources I1 and I2 are connected to drain terminals of metal-oxide semiconductor field-effect transistors (MOSFETs) M1 and M2 respectively, and constant current sources 13 and 14 are connected to source terminals of the MOSFETs M1 and M2 respectively. Gain adjustment resistors (variable resistors) R3 and R4 are bridge-connected between source terminals of the MOSFETs M1 and M2. Gate terminals of the MOSFETs M1 and M2 are connected to differential input terminals IP and IM, respectively. The drain terminals of the MOSFETs M1 and M2 are connected to differential output terminals OP and OM, respectively.
FIG. 2 shows a circuit diagram of an AGC amplifier including the conventional OTA. An AGC gain is set based on setting in resistors (variable resistors) R3 and R4 inside an OTA 50 and external resistors (variable resistors) R5 and R6. If there is a difference in characteristic between a pair of transistors (referred to as differential pair) that forms differential input stages of the OTA 50, then a DC offset voltage is produced, causing an output error.
Conventionally, a high pass filter (HPF) including capacitors C1 and C2 and resistors R1 and R2 is disposed in an output stage of the AGC amplifier in order to remove the DC offset voltage. In such a configuration, source follower circuits are disposed as a buffer that drives the capacitors C1 and C2 included in an output section.
FIG. 3 shows a circuit diagram of a source follower circuit. In FIG. 1, a source follower circuit including M1 or M2 satisfies the following relations:Vout/Vin=1  (1)Vout=Vin−Vth=Vin−0.6  (2)Vout>VDS2  (3)
Therefore, the following relation is obtainedVin>VDS2+0.6  (4)                where Vth is a threshold voltage.        
When the source follower circuit is used, a voltage shift equivalent to a gate-source voltage (VGS) in an N channel-MOSFET is caused in the output of the AGC amplifier. The range of the input voltage is expressed as, for example, Vin>VDS2+0.6 (V) according to the expression (4). If the AGC amplifier 50 is provided with a circuit configuration having a low power supplied voltage, then the amplitude (dynamic range) of the output voltage cannot be made wide under the influence of the voltage shift, resulting in a problem.